1. Field of the Invention
The invention relates to an optical sensor for measuring physical parameters on industrial components and structures, in particular on rotor blades of wind power plants.
2. Description of the Relevant Art
DE 10 2005 0016524 discloses a wind power plant in which operating states or anomalies on the rotor blades are determined by means of strain gauge sensors. Electrical sensors like these strain gauge sensors have the problem of being sensitive to electrical or electromagnetic disturbances. Thus, such sensors can be disturbed or destroyed by the discharge of electrostatic charges on the rotor blades or by lightning strikes.
U.S. Pat. No. 4,773,753 discloses a fiber sensor which can be used to measure temperatures or strains. The sensor is based on a polarisation-maintaining fiber. External influences such as temperature or strain alter the polarisation of light transmitted through the fiber. At the end of the fiber, the polarisation of the light is evaluated in order to obtain information about the variables to be measured.
The use of optical sensors allows independence of electrical fields and waves to be achieved in this case. Drawbacks of using optical fiber-based sensors include their complex handling and their high mechanical sensitivity. Furthermore, sensors based on the evaluation of change in polarisation have in many cases only low resolution or accuracy.
Another problem of sensors of this type, which can measure a plurality of measurement variables such as temperature and strain simultaneously, is distinguishing which measurement variable the measurement signal was generated from. If, for example, a change in polarisation is detected, then it is not possible to unambiguously ascertain whether the change was caused by an alteration of the length of the fiber or by a change in temperature.
A sensor based on a Bragg grating, such as is disclosed in U.S. Pat. No. 7,027,672 B2, offers a higher resolution. Drawbacks of this sensor include the typically low bandwidth of the evaluation unit, so that only slow measurements in the range of a few hertz are possible. In addition, the evaluation unit is very complex and cost-intensive.
EP 0564366 A1 describes an optical fiber sensor for the temperature-compensated measurement of a mechanical deformation. The phase difference of the signals is evaluated from two measurement fibers.